Often described as "the map is not the territory", this mental model refers to the relationship between an object and a representation of that object. A map is an abstraction and imperfect representation of a territory--it is not the territory itself. Abstractions are a necessary step in simplifying the complexities of reality. After all, imagine how difficult it'd be to drive in an unknown area without a map. However, we should be aware that these abstractions have limitations and are created using assumptions that may not always be true. This applies to mental models as well--while useful in explaining common situations, reality is not bound to conform to these models.
Newton's laws of motion are a model explaining physical phenomena. While an excellent approximation for most cases, these laws do not hold when working at very small scales, at very high speeds, or in very strong gravitation fields. These cases require different models.
The Treachery of Images is a famous painting of a pipe with a caption stating this is not a pipe. In describing it, painter René Magritte said, "...could you stuff my pipe? No, it's just a representation, is it not? So if I had written on my picture 'This is a pipe', I'd have been lying!"
This classic clip from The Office of Michael Scott driving into a lake is a literal example of the map not being the territory.